Propellant for fracturing wells

ABSTRACT

An apparatus for fracturing wells employs a propellant charge with a metallic foil to rapidly ignite the surface of the propellant charge. The assembly can be covered with an protective coating to protect against fluids in the well bore.

RELATED APPLICATION

The present application is based on, and claims priority to the Applicant's U.S. Provisional Patent Application No. 60/618,248, entitled “Propellant for Fracturing Wells,” filed on Oct. 13, 2004, and U.S. Provisional Patent Application No. 60/621,693, entitled “Propellant for Fracturing Wells,” filed on Oct. 25, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of well fracturing. More specifically, the present invention discloses a propellant assembly for fracturing wells.

2. Statement of the Problem

Propellant charges have been used for many years to create fractures in oil, gas and water-bearing formations surrounding a well. FIG. 1 is a cross-section diagram of a well 10 with a packer 12 and a series of propellant charges 20. The propellant charges 20 are ignited to rapidly generate combustion gases that create sufficient pressure within the well bore to generate fractures in the surrounding strata.

In order to achieve proper pressure-loading rates and adequate minimum pressures for sustained periods of time sufficient to extend fractures in the surrounding formations using gas-generating propellants, it is necessary that a sufficient surface area of propellant be burning to generate the volume of gas required to extend such fractures, as gas generation is a function of the surface area of the propellant burning at any given time.

Typical ignition systems for propellant incorporate detonating or deflagrating materials in a cord-like format. Such ignition systems, however, ignite only small areas of the propellant immediately adjacent to the detonating or deflagrating cord. In addition, detonating cords tend to have two problems: (1) if too brisant, the cord tends to shatter the surrounding propellant, resulting in initial burn areas that are unknown and difficult to model; and (2) the cord only ignites small areas immediately adjacent to the cord, relying on flame spread to initiate adjacent surface areas. Deflagrating cords have limited burn rates (on the order of 1000 ft/sec) that are insufficient to ignite large areas of the propellant surface within a multi-millisecond time frame.

If ignition of the propellant is limited to small areas of the propellant surface, the flame from the initial burning area of the propellant must spread across the face of the propellant to ignite the remaining surface area. This flame spread rate is a key limiting factor to achieving proper pressure loading rates and adequate minimum pressures for fracture propagation in the surrounding formations. If the flame spread from a localized ignition point is too slow, then the burning surface area at any given point in time will be limited, and the overall time that the propellant burns to completion may have to be extended sufficiently to compensate for the reduced amount of time that pressures exceed the minimum required fracture extension pressure, resulting in a longer but less efficient propellant burn.

In addition, the propellant burn should be predictable and reproducible for the purpose of accurately modeling the fracturing process. It is difficult to accurately model a propellant burn unless the entire exposed surface of the propellant is ignited almost simultaneously. Modeling of propellants has been contemplated in the past, but with the assumption that ignition of the propellant surface over the entire exposed area of the propellant is simultaneous. Practically speaking, such simultaneous ignition is difficult to achieve.

The problem is further complicated by the presence of well fluids. When propellants are submerged in well fluids such as water (or water and potassium chloride), flame spread rates tend to decrease. In addition, certain chemical coverings that are used as surface coatings on propellants to prevent leaching of the propellant fuel oxidizers into the surrounding well fluids also tend to inhibit the flame spread rate, thus exacerbating the problem. When such coatings are not applied to the surface of the propellant, sufficient leaching of the fuel oxidizer can take place over relatively short periods of time (i.e., about 1 hour) to result not only in a reduction in the available energy to do work on the formation, but also create an outer boundary layer absent of fuel oxidizer and comprised primarily of the propellant binder, which tends to inhibit the flame spread rate because the exposed fuel oxidizer in the binder has been leached away. Furthermore, because gas generation is a function of the area of propellant burning at any given time, it is also useful to engineer a propellant fracturing system that accounts for the required initial burning surface area to provide adequate pressure rise, in addition to taking into account the flame spread rate.

In addition, it would be preferable to configure the propellant such that there is a rapid decrease in the burning surface area, rather than a slow regressive decrease in area to maintain the pressures above that of the fracture extension pressure as long as possible. This provides the most efficient use of the available bond energy of the propellant that is burned in the well.

In summary, the prior art has the following shortcomings:

-   -   Detonating cord does not ignite sufficient surface area and         relies on flame spread.     -   Detonating cord, if made too energetic to overcome the limited         ignition area problem, can be too brisant and may shatter the         propellant, resulting in an unknown burning surface area.     -   Flame spread is too slow to achieve adequate burning surface         area of propellant for proper loading rate to cause multiple         fractures.     -   Slow flame spread results in slow pressure rise, increasing heat         loss by conduction into the surrounding well fluids, reducing         the useful work available to extend fractures.     -   Insufficient burning surface areas do not result in generated         pressures above that of fracture extension, limiting         effectiveness.     -   Burning rate and flame spread are limited when the propellant is         surrounded by well fluids.     -   Sealers tend to inhibit the flame spread.

Solution to the Problem. One solution to address the problems discussed above is to rapidly ignite the entire surface of the propellant charge by means of a metallic foil (e.g., a bimetallic nickel-aluminum, nickel-palladium, or nickel-zirconium foil) in order to produce a burn that is reproducible, and can be accurately modeled to predict the resulting conditions in the well and surrounding strata during the fracturing process.

SUMMARY OF THE INVENTION

This invention provides an apparatus for fracturing wells that employs a propellant charge with a metallic foil to rapidly ignite the surface of the propellant charge. The resulting rapid ignition of the propellant surface can be modeled more accurately and results in a more efficient use of the propellant charge in fracturing the well.

These and other advantages, features, and objects of the present invention will be more readily understood in view of the following detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be more readily understood in conjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional diagram of a well 10 with a packer 12 and a series of propellant charges 20.

FIG. 2 is a cross-sectional view of a propellant assembly prior to ignition.

FIG. 3 is a cross-sectional view of the propellant assembly in FIG. 2 after ignition.

FIG. 4 is a cross-sectional view of another embodiment of the propellant assembly prior to ignition.

FIG. 5 is a cross-sectional view of the propellant assembly in FIG. 4 after ignition.

FIG. 6 is a side cross-sectional view of another embodiment of the propellant assembly.

FIG. 7 is an orthogonal cross-sectional view of the embodiment of the propellant assembly shown in FIG. 6.

FIG. 8 is a top view of another embodiment of the propellant assembly using a series of bimetallic ignition strip fuses 80 to ignite the metallic foil 30. A portion of the outer protective layer 40 has been removed to show the ignition strip fuses 80 and foil 30.

FIG. 9 is an orthogonal cross-sectional view of the embodiment shown in FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

Turning to FIG. 2, a cross-sectional view is shown of one embodiment of the propellant assembly prior to ignition. The propellant charge has been divided longitudinally into two segments 20 a and 20 b having opposing interior surfaces. Alternatively, the propellant charge could be divided into thirds, quarters, or any other desired fractional shape. The outer surface of the combined segments of the propellant charge 20 a, 20 b has a generally cylindrical shape with dimensions suitable for insertion into a well bore.

A metallic foil 30 is sandwiched adjacent to the interior surfaces of propellant segments 20 a, 20 b and optionally around the circumference of the propellant to ignite these surfaces. Bimetallic foils 30 have been demonstrated to generate enough heat to ignite the surfaces of the propellant segments 20 a, 20 b. For example, a bimetallic foil having a thickness of approximately 30 microns made of nickel-aluminum, nickel-palladium or nickel-zirconium has been found to be suitable, although other metallic foils could be substituted. It should be understood that a thin metallic mesh could also be substituted, and should be interpreted as falling with the scope of a “foil” for the purposes of this invention.

An ignition element 50 is employed to initially ignite the metallic foil 30. Preferably, an extremely mild detonating cord is used as the ignition element 50. The detonating cord is sufficiently mild to not shatter the foil 30 or propellant segments 20 a, 20 b, yet it ignites the metallic foil 30, which in turn ignites the propellant segments 20 a, 20 b. For example, a mild detonating cord having 2.5 grains per foot of HNS explosive sheathed in lead could be employed. The detonating cord 50 can be ignited conventionally (e.g., with an igniter patch). The detonating cord 50 can either be enclosed in a metal sheath (e.g., a lead or mild steel tube), or placed directly in contact with the foil 30. Mild detonating cord is also commercially available with various metal sheathes, such as silver, aluminum or tin.

The propellant 20 a, 20 b is configured to directly contact the metallic foil 30 such that it maximizes the exposure to the fuel oxidizer component of the propellant 20. The mild detonating cord has a burn rate of approximately 17,000-20,000 ft per second, and thus the metallic foil 30 is ignited along the area adjacent to the mild detonating cord 50 within approximately 2.5 milliseconds for a practical-sized propellant treatment (less than 50 ft). Note that most propellant treatments are in the range of 10 to 20 ft., reducing this initiation time to less than 1 millisecond. The metallic foil 30 is then ignited. Because the foil 30 ignites all or nearly all of the exposed interior surfaces of the propellant segments 20 a, 20 b, and because the distance that the foil 30 must ignite is limited to the approximate radius of the propellant charge 30, the burn rate of the foil 30 is not as critical as the detonating cord 50. Furthermore, the propellant area adjacent to the foil 30 can be roughened by cutting, rather than extruded, thus exposing more fuel oxidizer to facilitate ignition. After the propellant 20 is burning, combustion gases 55 generated from the burn are directed as shown in FIG. 3, thereby preventing any well fluid from entering the area of burn. This allows the propellant 20 to establish and maintain a rapid burn.

Alternatively a rapid deflagrating cord could be employed in place of detonating cord, although rapid deflagrating cord has a much slower speed on the order of about 1000 ft/sec. Both detonating cord and deflagrating cord should be considered as examples of the types of the ignition elements that could be employed.

The entire propellant assembly can be wrapped or sealed in a protective layer or coating 40 as depicted in the cross-section view provided in FIG. 2. The propellant assembly can be wrapped in a water-tight aluminum scrim, heat shrink plastic, or other similar materials. For example, the propellant assembly can be wrapped with a polymeric or fluoroelastomeric shrink-wrap material, such as the VTN-200 material marketed by the 3M Corporation of St. Paul, Minn.

The protective layer 40 serves to protect the propellant assembly during transportation, handling, and insertion into the well bore. In particular, the protective layer 40 keeps all propellant and related ignition components dry, thus reducing leaching and eliminating the requirement to apply a sealer. It also compresses the propellant segments 20 a and 20 b against the foil 30, facilitating heat transfer and ignition. Thus, there is little inhibition to flames spreading along the surfaces of the segments 20 a, 20 b of the propellant charge. FIG. 3 is a cross-sectional view of the propellant assembly in FIG. 2 after ignition. The sharp increase in pressure resulting from the combustion gases produced by the propellant charge 20 ruptures the protective layer 40. As a result, sufficient surface area can be rapidly initiated as required to provide controlled pressure loading and sustained to assure fracture extensions which result in more efficient use of the propellant bond energy for improved production that would result from such multiple fractures and their extension.

An alternative embodiment of the propellant assembly is shown in FIGS. 4 and 5 with the detonating cord 50 in a groove on the outer surface of the propellant charge 20. A protective coating 40 covers both the detonating cord 50 and propellant charge 20 to keep them dry. FIG. 4 is a cross-sectional view of the propellant assembly prior to ignition and FIG. 5 is a cross-sectional view of the propellant assembly after ignition.

An additional embodiment of the propellant assembly is shown in FIGS. 6 and 7 with metallic foil 30 covering the exterior surface of the propellant charge 20. FIG. 6 is a side cross-sectional view of this embodiment of the propellant assembly and FIG. 7 is an orthogonal cross-sectional view. A protective layer 40 covers the foil 30 and propellant charge 20. The metallic foil 30 adjacent to the exterior surface of the propellant charge 20 is ignited by a small piece of propellant 53 at one or more locations in the assembly. A number of channels or grooves 25 in the exterior surface of the propellant charge 20 can be used to facilitate the spread of hot combustion gases from the ignition element 53 over large areas of the metallic foil 30.

For example, a dowel or rod 53 of propellant could be used for this purpose as the ignition element for the foil 30. The propellant dowel 53 is ignited by a shaped charge igniter 51 that fires through an isolating bulkhead 52 into the top of the propellant assembly. The propellant dowel 53 then ignites producing a burst of hot gas that is oriented directionally along the channels 25 down the longitude of the propellant charge 20. This burst of hot gases produces temperatures over large areas of the foil 30 sufficient to ignite the foil 30 very rapidly. In turn, the foil 30 rapidly ignites the exterior surface of the propellant 20 beneath the protective layer 40. The protective layer 40 is distended and then ruptured by the internal pressure created by these combustion gases.

Alternatively, the metallic foil could be ignited electrically using capacitors in an electrical circuit to create the required power output to simultaneously ignite bimetallic ignition strip fuses 80 that in turn ignite the metallic foil 30 at multiple locations. FIG. 8 is a top view of this embodiment with a portion of the outer protective layer 40 removed to show the ignition strip fuses 80 and metallic foil 30. FIG. 9 is an orthogonal cross-sectional view of the embodiment shown in FIG. 8. Positive and negative wire braid conductors 82 and 83 are connected to the electrical power source and run longitudinally along the propellant assembly. These leads 82, 83 are diametrically-opposed to one another and are shown at the top and bottom in FIG. 9. The metallic foil 30 does not completely cover the circumference of the propellant charge 20, but rather leaves two narrow, diametrically-opposed gaps beneath the conductors 82, 83. As shown in FIG. 9, this results in an electrical path running from the upper conductor 82 through the upper set of ignition strip fuses 80, both sides of the metallic foil 30, and the lower set of ignition strip fuses 80 to the lower conductor 83. The electrical current through the ignition strip fuses 80 causes them to ignite, which in turn ignites the metallic foil 30 and the propellant 20. The ignition strip fuses 80 can be located at selected intervals along the length of the propellant assembly, as shown in FIG. 8, to achieve a desired pressure rise. The capacitors can be charged by the wire line conveying device, or batteries in tubing conveyed applications.

The above disclosure sets forth a number of embodiments of the present invention described in detail with respect to the accompanying drawings. Those skilled in this art will appreciate that various changes, modifications, other structural arrangements, and other embodiments could be practiced under the teachings of the present invention without departing from the scope of this invention as set forth in the following claims. 

1. An apparatus for fracturing wells comprising: a propellant charge for insertion into a well and having at least one surface; a metallic foil adjacent to the surface of the propellant charge; and an ignition element for igniting the metallic foil, and thereby rapidly igniting the surface of the propellant charge.
 2. The apparatus of claim 1 further comprising a protective layer covering at least a portion of the propellant charge.
 3. The apparatus of claim 2 wherein the protective layer comprises a fluoroelastomeric material.
 4. The apparatus of claim 1 wherein the ignition element comprises a piece of propellant generating hot combustion gases to ignite the metallic foil.
 5. The apparatus of claim 1 wherein the metallic foil comprises nickel and aluminum.
 6. The apparatus of claim 1 wherein the metallic foil comprises nickel and palladium.
 7. The apparatus of claim 1 wherein the metallic foil comprises nickel and zirconium.
 8. The apparatus of claim 1 wherein the propellant charge is divided into a plurality of segments with interior surfaces between adjacent segments, and wherein the metallic foil is sandwiched between the interior surfaces.
 9. The apparatus of claim 1 wherein the propellant charge has a substantially cylindrical exterior surface covered by the metallic foil.
 10. An apparatus for fracturing wells comprising: a propellant charge for insertion into a well, said propellant charge being divided longitudinally into a plurality of segments with interior surfaces between adjacent segments; a metallic foil between the interior surfaces of the propellant charge; and an ignition element for igniting the metallic foil, and thereby rapidly igniting the interior surfaces of the propellant charge.
 11. The apparatus of claim 10 further comprising a protective layer covering at least a portion of the exterior of the propellant charge.
 12. The apparatus of claim 11 wherein the protective layer comprises a fluoroelastomeric material.
 13. The apparatus of claim 10 further comprising a longitudinal passageway extending from the ignition element along an interior surface of the propellant charge for propagation of combustion gases along length of the propellant charge.
 14. The apparatus of claim 13 wherein ignition element comprises a dowel of propellant in the longitudinal passageway.
 15. The apparatus of claim 10 wherein foil comprises nickel and aluminum.
 16. An apparatus for fracturing wells comprising: a propellant charge for insertion into a well and having an exterior surface; a metallic foil covering the exterior surface of the surface of the propellant charge; a protective layer covering the metallic foil and propellant charge; and an ignition element for igniting the metallic foil, and thereby rapidly igniting the surface of the propellant charge.
 17. The apparatus of claim 16 wherein the metallic foil comprises nickel and aluminum.
 18. The apparatus of claim 16 wherein the protective layer comprises a fluoroelastomeric material.
 19. The apparatus of claim 16 wherein the propellant charge further comprises a groove in the exterior surface of the propellant charge containing the ignition element. 